This invention particularly relates to an improved process for dewaxing refined vegetable oils and, more particularly, to a process in which consistently efficient separation of undesirable waxes, particularly high-melting fatty acid esters, can be achieved for oils such as sunflower, safflower, and corn oil for refrigerator clarity stability. Vegetable oils, and more particularly edible vegetable oils, are used as salad oils, cooking oils, margarine constituents, and the like. Safflower, sunflower and corn oil, in particular, are often used in these and other food applications.
For purposes of this application the terms "wax" and "high-melting material" are used interchangeably and are intended to be generic to the many substances which can cause clouding in refined liquid vegetable oils at temperatures of less than about 60.degree. F. (15.5.degree. C.).
A problem which arises in the purification of vegetable oils is that the crude oils tend to have high contents of undesirable insoluble material. Natural vegetable seed oils are composed of mixtures of many naturally produced chemical compounds, including not only the oily constituents but also, usually, small percentages of natural phosphatides, vegetable waxes, pigments, and many other compounds. The oily constituents, namely, the glyceride esters of the long chain fatty acid of the saturated and unsaturated types, make up the largest fraction of vegetable oils. Such materials to a large extent determine the properties of the oil, but the remaining constituents also exert a marked and sometimes detrimental effect, depending upon the use to which the oil is put.
Some natural vegetable oils of commercial grade, such as safflower, sunflower, cottonseed oil, soybean oil, peanut oil and corn oil, become cloudy after having been chilled or cooled to somewhat lower than room temperatures, as in a refrigerator, and remain cloudy when returned to ordinary (room) temperatures. The cloudiness may be followed by a settling out of an opaque layer, a particularly disadvantageous occurrence when the oil is packed in glass and where clarity and brightness are of importance.
A large portion of the high-melting material can be removed from oils by a process known as "winterizing" in which the oils are carefully cooled to low temperatures for extended periods of time to permit precipitation of solid material. Solid material can then be removed by pressing or other separation procedures. However, not all of the high-melting solid material is removed from oils by winterizing, and the oil still tends to cloud when stored for extended periods of time at low temperature. Moreover, the usual winterizing treatment undesirably tends to remove by entrainment a substantial portion of the olein fraction of the oil.
Many previous attempts have been made to remove wax fractions from crude vegetable oils by mechanical separation, such as centrifuging and filtration at low temperatures. These techniques have been somewhat unsuccessful due in part to the small differences in specific gravity between the wax and the vegetable oil and also due to the compaction of the vegetable oil wax under pressure in a filter to a slime or grease consistency which resists the passage therethrough of the vegetable oil.
U.S. Pat. No. 4,035,402 to Levine, July 12, 1977, discloses a process for effectively dewaxing vegetable oils which comprises: "chilling a refined and water-washed or filtered vegetable oil, mixing the chilled vegetable oil with a dilute alkaline solution, gently agitating the resulting mixture, centrifuging the mixture and separating a heavy phase which contains wax and other impurities from a light phase which may then be bleached and deodorized to form a clear oil".
It has also been suggested to employ electrofiltering techniques for the removal of suspended particulate matter in the oils, both with and without some type of media disposed between electrodes. For example, a method and apparatus for removing electrically conductive suspended contaminants from high-resistivity oils free of significant amounts of dispersed water, disclosed in U.S. Pat. No. 3,928,158 to Fritsche et al, Dec. 23, 1975, incorporated herein by reference in its entirety, particularly its teachings with respect to the electrofilter apparatus.
U.S. Pat. No. 2,054,075, Fisher, teaches chilling wax-bearing inedible oil in the presence of a suitable diluent containing precipitated wax. Fisher teaches "shock chilling" the oils at -20.degree. F. and -40.degree. F. Similarly, U.S. Pat. No. 2,617,763, Fischer, teaches an electrically aided filter system for removing paraffinic waxes from inedible oil: "For example, wax crystals in propane-deasphaltized residual oils in a nonconductive dichlorethane solvent, chilled to -25.degree. C. by a conventional method, are smaller in average size than the diameter of the openings in the weave formed by the electrical conductors 14 and 15. Obivously, the filter structure without an electrical charge will allow part of the crystals to pass through the filter. However, by placing a charge of only a dozen to a few hundred volts, the crystals are retained by the filter and the filtrate pour point is improved by 4.degree. F. to 10.degree. F. with the application of from approximately 50 to 500 volts." Col. 3, lines 30-45.
Another patent, U.S. Pat. No. 2,302,386, Fisher, teaches on page 3, lines 43-49 (rhc) of that patent that: "Rapid chilling such as this is known as `flash chilling` or `shock chilling` and apparently results in the formation of a finer wax precipitate than is formed by slower cooking methods. The formation of such fine precipitates is advantageous in the electrical dewaxing process." (Emphasis added.)
While such methods may be classified in the same general class as that of the present invention, it is quite evident to those skilled in the art that neither the Levine nor the electrofilter patents teach the advantage of a slow controlled tempering step before electrofiltration for the removal of high-melting fatty acid esters and similar contaminants from refined vegetable oils to provide refrigerator clarity stable oils. It will also be understood that while prior electrofilter techniques have been reasonably effective when dealing with conductive contaminants, such as certain hydrogenation catalysts, a different problem is posed when handling nonconductive waxes and similar contaminants in refined vegetable oil.